And I showed the pictures to Professor Poliakoff from the Periodic Table of Videos... He said: "The circular depressions suggest to me that it might be slag from a blast furnace. Slag is quite light and has holes from the gas bubbles coming out of the process. However, I'm not a mineral expert."

I also showed our cult hero technician Neil Barnes, who said he'd like a bit of the sample sent to him for testing!

Many people have asked for more information about the rock to help with identification.

The viewer (a chap named Matt) has supplied these details:

Coin used in picture is Canadian 25 cent.Does not float in water.It is not magnetic at all.It was found on St. James Street, in Sault Ste Marie Ontario, Canada.I have measured it at 7 centimeters long, and 3 centimeters wide.It leaves no visible coloration if I scratch a piece of paper with the rock.It is very hard, as I tried to use a metal file to scratch it but it barely even made a mark.The very few parts that did come off, did not dissolve in water, nor did they change color. I unfortunately do not have hydrochloric acid, nitric acid or sulfuric acid to test with, sorry.

Perhaps the best advice so far came from someone on Twitter who suggested Matt "ask his local university's geology/chemistry department".